1. Field of the Invention
The disclosed technology relates to the field of electron paramagnetic resonance. More particularly, the technology relates to methods and systems for applying pulsed electron paramagnetic resonance.
2. Description of the Related Technology
Magnetic resonance techniques are widely spread and find their application in amongst others detection and imaging. Whereas continuous working often may be inefficient as during a magnetic field scan most of the time baseline data is recorded between resonance responses, pulsed magnetic resonance typically uses the measurement time to a better effect. In pulsed magnetic resonance an impulse excitation is applied at the resonant frequency and the Fourier transform of the resulting free induction decay signal reveals the spectrum. The pulsed magnetic resonance technique may result in sensitivity advantages. In pulsed electro paramagnetic resonance (EPR) measurements, a short pulse is applied on a probe after which the magnetization of the probe is measured.
One problem with pulsed EPR is that the ring-down of the antenna caused by de-phasing of the spins, lasts longer than the free induction decay (FID), i.e. the signal to be detected, coming from the probe. This FID is longer when the line width of the probe is smaller. It consequently has been found previously that pulsed EPR should be limited to probes with a line width smaller than 3 MHz or that for pulsed EPR it is desirable to have spin probes, which have a very narrow single line spectrum because the transverse relaxation time is inversely proportional to line width. The problem of pulsed EPR for probes with a broad line width is shown in FIG. 1. FIG. 1 illustrates the initial alignment of spins with a static magnetic field along the Z-axis (part (a)), the subsequent alignment of the spins along the Y-axis after a Π/2 pulse, which would allow capturing of the probe signal by the antennas (part (b)), and the de-phasing of the spins resulting in the probe signal being not available (part (c)).
Some solutions have been provided to measure a short probe signal without being influenced by the ring-down time of the antennas. One known technique is the use of the Hahn echo. Using the Hahn echo, a first pulse will flip the spins 90 degrees with respect to the static magnetic field. During a period of time the tilted spins will de-phase (each with its own frequency), after which a second pulse will tilt the spins 180 degrees. Due to this pulse, the spins will re-phase. They will be re-phased exactly after a time equal to the time between the pulses. At this moment the echo can be measured.
It is also known to use pulse trains as a preparation technique for experiments. Some techniques are known for obtaining material characteristics wherein a set of three pulses is first used for creating a stimulated echo, and whereby a response of the material is induced by a re-phasing pulse. Other techniques use variations of applying pulses or pulse trains, but these all use a re-phasing pulse to induce a response of the material.
It is known to use shaped sinc pulses to achieve a more uniform power excitation over a relatively wide bandwidth region. For example, a truncated sinc pulse may be used to compensate for the Q-profile of the antenna.
It is also known, for techniques applying a pulse sequence, to correlate the input sequence with the measured result. One way to perform this is to change the phase between the pulses to eliminate the overlap of signals after multiple pulse sequences. Another example is correlating FID with the input sequence, to decrease the total acquisition time and to improve the signal to noise ratio of the FID signal.